Doping of semiconductor substrates with conductivity-determining type elements, such as n-type and p-type elements, is used in a variety of applications that require modification of the electrical characteristics of the semiconductor substrates. Photolithography is a well-known method for performing such doping of semiconductor substrates. Photolithography requires the use of a mask that is formed and patterned on the semiconductor substrate. Ion implantation is performed to implant conductivity-determining type ions into the semiconductor substrate in areas corresponding to the mask. A high-temperature anneal then is performed to cause the ions to diffuse into the semiconductor substrate.
In some applications such as, for example, solar cells, it is desirable to dope the semiconductor substrate in a pattern having very fine lines or features. The most common type of solar cell is configured as a large-area p-n junction made from silicon. In one type of such solar cell 10, illustrated in FIG. 1, a silicon wafer 12 having a light-receiving front side 14 and a back side 16 is provided with a basic doping, wherein the basic doping can be of the n-type or of the p-type. The silicon wafer is further doped at one side (in FIG. 1, front side 14) with a dopant of opposite charge of the basic doping, thus forming a p-n junction 18 within the silicon wafer. Photons from light are absorbed by the light-receiving side 14 of the silicon to the p-n junction where charge carriers, i.e., electrons and holes, are separated and conducted to a conductive contact, thus generating electricity. The solar cell is usually provided with metallic contacts 20, 22 on the light-receiving front side as well as on the back side, respectively, to carry away the electric current produced by the solar cell. The metal contacts on the light-receiving front side pose a problem in regard to the degree of efficiency of the solar cell because the metal covering of the front side surface causes shading of the effective area of the solar cell. Although it may be desirable to reduce the metal contacts as much as possible to reduce the shading, a metal covering of approximately 5% remains unavoidable since the metallization has to occur in a manner that keeps the electrical losses small. In addition, contact resistance within the silicon adjacent to the electrical contact increases significantly as the size of the metal contact decreases. However, a reduction of the contact resistance is possible by doping the silicon in narrow areas 24 directly adjacent to the metal contacts on the light-receiving front side 14 with the dopant of opposite charge of the basic doping, thus creating a selective emitter.
The fabrication of a selective emitter, comprising the heavily-doped narrow areas 24 and the relatively lightly-doped areas 26 adjacent to the areas 24, traditionally requires several processing steps. In particular, the fabrication of a selective emitter, or the fabrication of any structure requiring a highly-doped area and a lightly-doped area, typically includes two doping steps and two diffusion steps. However, such methodology is time consuming and costly.
Accordingly, it is desirable to provide methods for forming two different doped regions in a semiconductor material that utilize only one diffusion step and, therefore, are time efficient. In addition, it is desirable to provide methods for fabricating semiconductor devices wherein the methods utilize one diffusion step to achieve two different regions of a semiconductor material having different dopant profiles of elements of the same conductivity-determining type. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.